1. Field of Invention
The invention relates to a compound inkjet print head printer, and particularly to a compound inkjet print head printer that operates with increased speed and high photo quality.
2. Related Art
A conventional inkjet printer ejects ink droplets at predetermined positions to form texts or images by moving its print head back and forth on a printing medium. The size, shape, material, concentration and position of the ink droplets from the print head are critical to the printing quality.
Printing photos by using inkjet printers is becoming more and more popular. Therefore, the printing resolution and color representation are increasingly more important. The resolution of an inkjet printer is measured by dots per inch of the printing medium. The higher the resolution, the finer the layout is with a smoother rim of the image. Furthermore, the color representation is based on gradations in a single pixel. A gradation means a level of color. High gradation has fine continuous levels of color and is rich in color variation. Hewlett-Packard has proposed photo resolution increasing technology for high-gradation photo printing. The print head repeatedly moves back and forth to eject one or more ink droplets at a single pixel according to color darkness. For example, if a dark red ink dot is wanted, then two or more red ink droplets are superimposed on the same dot. To prevent the resolution from being affected by the multiple ink droplets at a single pixel, the position of the ink droplets, the size of the ink droplets, and the number of ink droplets usually has to be accurately controlled, and the print head has to be compatible with the printing system. The smaller the ink droplets and the finer the pitch between ink droplets, the clearer the formed image. The image resolution is thereby increased and the gradation is much closer to reality. Generally, the smaller the ink droplets, the greater the number of ink droplets to fill up the pixel. As a result, the number of movements of the print head back and forth on the printing medium is increased, while the printing speed is lowered.
The problem of lower printing speed has been solved by changing the structure of the print head so that different sizes of ink droplets are ejected. For example, U.S. Pat. No. 6,020,905 discloses a thermal bubble type print head in which a heater is mounted to allow the print head to eject different sizes of ink droplets. However, mounting the heater in the print head increases the structural complexity and the production cost of the print head. U.S. Pat. No. 6,322,185 controls the voltage of a piezoelectric print head by a printing system to generate different amounts of piezoelectric vibration to eject ink droplets of different sizes. The above disclosures involve ejecting different sizes of ink droplets in different swaths, which limits the increase in printing speed.
Another solution is to provide nozzles of different sizes on the print head. For example, U.S. Pat. No. 5,412,410 forms two sets of jet holes on one print head to provide various sizes of ink droplets. Such a print head has a complex structure, such as at least two different sizes of jet holes, and actuators are mounted on one surface of the print head to provide different sizes of ink droplets simultaneously so as to increase the printing speed. However, for a given print area, various sizes of jet holes are arranged on the print head with a limited number. This increases the production and design cost while failing to effectively increase the printing speed and resolution.